1. Field of the Invention
The invention relates to a mixer and more particularly to a mixer with a self-calibrating carrier leakage mechanism.
2. Description of the Related Art
Wireless communication has been widely applied in various applications. Typically, each wireless communication device requires a radio transceiver (i.e., a receiver and a transmitter). With the demand for enhanced performance, a direct conversion technology has been developed for size, cost and lower power consumption. In direct conversion transmitters, however, LO pulling, RF carrier leakage due to I-Q DC offset, and I-Q gain/phase imbalance problems must be overcome.
FIG. 1 is a circuit diagram illustrating the leakage current effect on the modulator outputs. Input stage 11 provides two differential input signals, Vp and Vn, to the gm stage 13. The local oscillator generates two differential signals at a carrier frequency. During the direct conversion modulation process, the carrier power is theoretically completely eliminated. But, in real practice, complete elimination is difficult to achieve. The unwanted carrier power, referred to as local oscillator leakage, reduces the performance of the mixer 12. When the bias currents of M1 and M2 are mismatched, the differential local oscillator signals, Lon and Lop, leak to the modulator output, RFoutp or RFoutn. Assuming the bias current of transistor M1 is (I+Δi), the bias current of transistor M2 is (I−Δi), the local oscillator signal LOn is −ALOCOS(ωLOt), and the local oscillator signal LOp is ALOCOS(ωLOt), hence
                    RFoutp        =                              {                                          LOp                ⁡                                  (                                      I                    +                                          Δ                      ⁢                                                                                          ⁢                      i                                                        )                                            +                              LOn                ⁡                                  (                                      1                    -                                          Δ                      ⁢                                                                                          ⁢                      i                                                        )                                                      }                    ⁢                      •L            load                                                  =                  2          ⁢          Δ          ⁢                                          ⁢          i          ⁢                                          ⁢          •          ⁢                                          ⁢                      A            LO                    ⁢                      COS            ⁡                          (                                                ω                  LO                                ⁢                t                            )                                ⁢                                    •L              load                        .                              
It is apparent that the output signal of the mixer 12 comprises unwanted term, ALOCOS(ωLOt), from the differential local oscillator signal LOp. For better performance, the carrier leakage should be minimized, thus, a calibration unit for reducing the Δi is desirable.
FIG. 2 illustrates a carrier leakage calibration system disclosed in U.S. Patent Application Publication US2004/0132424. The leakage detector 22 receives the output signal of direct up transmitter 21 as a feedback signal, detects carrier leakage, and transmits the detection result to the correction algorithm 23. The correction algorithm 23 receives the detection result from the leakage detector 22 to generate a digital value to the digital to analog converters (DAC) 24 and 25. The DACs 24 and 25 respectively outputs compensation in-phase modulation signal Io(t) and compensation quadrature modulation signal Qo(t) to in-phase modulation signal I(t) and quadrature modulation signal Q(t) to achieve the carrier leakage calibration.
FIG. 3 is a block diagram of a direct up-conversion transmitter with a compensation feedback path in U.S. Patent Application Publication US2003/0045249. The impairment detector 31 measures signal impairments in the direct up-converter output 33 and generates a feedback signal 34 to the impairment compensator 32. The baseband processor 35 generates in-phase (I) and quadrature-phase (Q) digital baseband signals for RF transmission. The I and Q baseband signals are modified prior to analog conversion by the impairment compensator 32. The modified baseband signals are then converted into the analog domain by the DACs 36. The direct up-converter 33 which combines the analog baseband signals with an RF carrier signal 37 from the frequency synthesizer 38. The feedback signal 34 is used by the impairment compensator 32 to pre-distort the I and Q baseband signals such that the pre-distortion cancels any actual distortion caused by impairments in the direct up-converter 33.